The drag reduction performance of a closely packed array of shallow diamond-shaped dimples was investigated experimentally in a fully developed turbulent channel flow. These dimples have a smooth rounded edge, a streamwise length of 100 mm, a spanwise width of 50 mm and a depth of 2.5 mm at their deepest point. The nature of their shape allows very efficient packing of the dimples in the array, and in the present study, a coverage ratio of about 99% of the total surface area was used. The diamond shape was obtained by deforming the basic circular dimple so that both the upstream and downstream ends are pointed. This deformation reduces the streamwise wall slope within the dimple, which resulted in less flow separation at the upstream dimple edge, and less flow impingement at the downstream edge. These two effects have considerable influence on reducing the form drag, and contributed to significant drag reduction of the diamond-shaped dimples compared to that of circular dimples. At low Reynolds numbers, the diamond-shaped dimples show a sharp reduction in drag as Reynolds number increases, leading to a maximum drag reduction of about 7.5% at a Reynolds number of about 20,000. Above this Reynolds number, drag increases but at a very gradual rate of about 0.17% per 10,000 increment in the Reynolds number.
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